In order to achieve high-speed transmission of large-volume data, there have been proposed systems based on information-communication equipment that utilizes a high-frequency range such as a microwave range of 1 to 30 GHz and a millimeter-wave range of 30 to 300 GHz. In addition, millimeter wave-using systems and the like such as a radar apparatus that measures inter-vehicle distance have been proposed to date.
As a circuit board structure adopted in such equipment utilizing a high-frequency range such as a microwave range and a millimeter-wave range, a monolithic microwave integrated circuit (MMIC) and a structure in which a passive electronic component is mounted on a planar circuit such as a microstrip line are known. In the planar circuit, constituent circuits such as a power divider circuit, a branch circuit, a matching circuit, a hybrid circuit, a filter circuit, and so forth are each put to its proper use to provide a circuit configuration capable of obtaining desired characteristics meeting system requirements.
Moreover, in order to effect radio wave transmission and reception in a circuit board, an antenna board mounting an antenna is required. For connection between the circuit board and the antenna board, in order to avoid mutual influences of high-frequency signals, the circuit board and the antenna board are commonly constructed independently of each other, for example, the antenna board is connected to the back surface of the circuit board. Moreover, in general, input-output ports disposed in the circuit board and the antenna board, respectively, are connected to each other through a waveguide.
For example, in Japanese Unexamined Patent Publication JP-A 2002-84208, there is disclosed a high-frequency module constructed by arranging a circuit board and an antenna board one above the other, interposing a waveguide adaptor between the circuit board and the antenna board, and connecting an input-output port at a side of the circuit board and an input-output port at a side of the antenna board to each other through a waveguide formed in the waveguide adaptor.
Moreover, when the input-output port is an opening of a waveguide, connection between the opening and the waveguide is established by a method of, for example, as disclosed in Japanese Unexamined Patent Publication JP-A 2002-185203, connecting the mutually-corresponding openings by a solder bump.
Moreover, in Japanese Unexamined Patent Publication JP-A 2004-254068, there is disclosed a high-frequency module in which an input-output port at a side of a circuit board and an input-output port at a side of an antenna board are connected directly to each other by means of a solder bump without using a waveguide adaptor or the like.
The capability of transmitting and receiving necessary high-frequency signals from a multitude of directions efficiently is demanded in a high-frequency module for use in a millimeter-wave radar apparatus or the like utilizing millimeter waves. Moreover, also in a wireless terminal apparatus that transmits images by exploiting millimeter waves, the capability of changing the direction of transmission and reception freely by taking advantage of the mobility of the wireless terminal is demanded. In order to fulfill such a capability, there is a need to impart, to an antenna board, a phased array antenna function, a digital beam forming function, or the like capable of electronically changing an antenna beam direction with increased directivity of an antenna board disposed in a high-frequency module. An antenna such as described above capable of effecting high-frequency signal transmission and reception in a multitude of directions can be implemented by using an array antenna which exhibits higher directivity with use of a plurality of antennas and a multi-port antenna constructed by arranging a plurality of array antennas.
However, in the high-frequency modules disclosed in JP-A 2002-84208 and in JP-A 2004-254068, the input-output port at the side of the circuit board and the input-output port at the side of the antenna board are provided on the opposed surfaces of the circuit board and the antenna board, respectively. Therefore, at the time of positioning of the input-output port and the waveguide or positioning of mutually-corresponding input-output ports, the input-output port is poorly visible. This leads to instability in positioning accuracy with consequent possibility of occurrence of significant positional deviation. In the event of such a positional deviation, there may be a case where local transmission-path variations take place at a connection part, for example, a transmission path over which a high-frequency signal is transmitted becomes narrow between the circuit board and the antenna board. In the local area of the transmission path subjected to variation, reflection of a high-frequency signal or the like occurs, which may give rise to a problem of an increase in transmission loss. Furthermore, the larger is the number of the input-output ports, the larger is the number of solder bumps which are used for connection of the input-output ports, with consequent complication of mounting process steps.